1. Field of the Invention
The present invention relates to AC to DC converter system with a ripple feedback circuit, and in particular, to a unity power factor, single phase rectifier system provided with a ripple feedback circuit to improve the AC current waveform and reduce the high frequency oscillation.
2. Summary of Related Art
The proliferation of line connected equipment using single phase rectifiers to process AC input power is a concern because of the poor power factor of the typical rectifier and the harmonic content of the AC line current of the rectifier.
Poor power factor results in poor utilization of the available current carrying capacity of the AC distribution system. High harmonic content in the line current frequently causes mutual interference among line-connected equipment. Distribution systems feeding a large rectifier or a large number of small rectifiers may experience overheating of transformers, interference with relaying and metering, and interference with other equipment. Because of this problem, standards agencies, such as the International Electrotechnical Commission, are preparing and promoting standards for harmonic reduction.
In response to the concerns noted above, a number of high power factor rectifier systems have been developed for single phase systems having power ratings as high as 10 kilowatts. Various techniques have also been developed to reduce rectifier harmonic levels for single phase operation. Passive filters on the AC power supply and choke input filters for the rectifier system have achieved only limited success in reducing harmonic levels and are bulky and expensive.
In computer power supplies and other applications requiring a low output ripple, voltage-sourced (boost-like) rectifier systems provided with two conversion stages are used to achieve reduced harmonic levels, low output ripple, and a high level of internally-stored energy for sustaining the system during a momentary power failure. The voltage-sourced rectifier systems are characterized by DC output voltages higher than the peak AC voltage and by complicated controls for stable operation. The voltage-sourced rectifier system is also a popular system because of the availability of economical capacitors to achieve the stored energy requirements at typical AC system voltages. However, voltage-sourced rectifier systems cannot be short circuit protected and may not be suitable at higher power ratings.
For applications able to utilize a modest ripple requirement, such as a DC motor drive, a single stage rectifier system is preferable for reducing harmonic levels provided short-circuit current limiting can be obtained. A current-sourced, buck-like rectifier system meets such a requirement.
The current-sourced rectifier system includes a large inductor to provide the required energy storage and can operate at open loop or can be made current limited down to zero output voltage. The maximum voltage for the current-sourced rectifier system is one-half the peak AC voltage. The current-sourced rectifier system is very effective in reducing harmonics and permitting full utilization of the semiconductors, even at medium and high power ratings. A costly and bulky inductor required by the current-sourced rectifier is the major drawback for the current-sourced system.
A resonant filter may be added to the current-sourced rectifier system to lower the peak stored energy, which consequently reduces the size and cost of the inductor. The resonant filter current-sourced rectifier system is also short circuit current limiting. One additional advantage of the current-sourced rectifier system is the ease in adding an isolation transformer to the system. No new conversion stages are required.
However, the use of a resonant filter with a current-sourced rectifier system results in two new problems. The AC line current distortion is quite sensitive to any distortion in the distribution system voltage. The line inductances typical of a weak AC system can also cause ringing or sustained oscillation of the rectifier.
Feedback of the ripple in the output current to the pulse width modulator used to control the gated switch increases the incremental output resistance of the converter to damp the oscillation, but it also deteriorates the AC line current waveform. The ripple feedback circuit of the present invention, minimizes the effect of these two problems.
The current-sourced rectifier system having a resonant load-balancing filter and ripple feedback circuit is not a universally useful solution, but in certain applications such a system will be very beneficial. The areas of greatest utility will be (1) applications requiring normal operation down to zero output voltage, or short circuit tolerance; (2) applications which can accept a current ripple of two percent or greater; and (3) applications which need galvanic isolation, using a high-frequency transformer, in a simple single stage power circuit.
Examples of applications for which the current-sourced rectifier system having a resonant load-balancing filter and ripple feedback circuit would be advantageous include DC motor drives (shunt or series), battery chargers, electroplating power supplies, welding power supplies, and arc discharge lighting power supplies.